Study On The Composition And Microstructure Change Of Reinforced Concrete Under Electric Field

Currently, rebar corrosion has been the most major factor influencing the durability of concrete structure.Therefore, the non-destructive electrochemical technology for protecting the bar has been taken a great deal of interests of researchers because of its time-saving, economical, constructing conveniently etc. generally non-destructive electrochemical protection technology could mainly be classified three types,cathodic protection, electrochemical chloride extraction and electrochemical realkalization, according to the different technology detail. However, these electrochemical protection technologies were all carried out by an external electric field set between rebar in concrete and an auxiliary anode on the surface of concrete to increase the OH-/Cl^- and make the steel surface into the passive potential.Though a plenty of study on electrochemical technology preventing corrosion were proved. Less of research on electron field influencing the composition and micro-structure of reinforced concrete were reported.So, in this paper,research on the microstructure variation of reinforced concrete,quantitative analysis the concentration distribution of Cl^-,Na⁺ and the corrosion state of steel surface after ECE(electrochemical chloride extraction) were carried out by which could be theoretical guidance and quantified the parameters of the electrochemical protection technology, which has been of importance to social and economic development .In this paper,a slab concrete model was established to analyze the migration and redistribution law of Cl^- under electric field. After selecting the appropriate initial and boundary conditions, the migration law of Cl^- was calculated by Nernst-Planck law; the Fick's second law and current balance It was found that the theoretically deduced result was fitted to the experimental one. The result shows that the distribution of chloride ions along the vertical plane of concrete is with a parabolic type curve profile under the effect of electric field, indicating that the maximum chloride concentration appears in concrete specimen. In pratical engineering, the concentration equation can be amended to quantitative analyse variation of ion concentration in different parts of the concrete structure then to guide the optimization parameters of electrochemical protection technology.Chemical composition and microstructure of concrete under the electric field were analyzed by XRD, DTA / TG, Hv, SEM etc. The results show that at the initial stage, the quantity of Ca(OH)₂ has increased with elapsed current time, and whose orientation was very high near the cathode, however, it will be weakened after exceeding a certain time.The micro-hardness of the interal embedded paste have exhibited the corresponding law with Ca(OH)₂, decreasing at the beginning stage and stabilizating at the point of Ca(OH)₂ orientation weakening. Scanning Electron Microscopy (SEM) photography showed that a amount of Ca(OH)₂ in the paste surface increases at the first stage, however, Sodium-rich silicate phases has been found at the later period. In general, the concrete zone near cathode were weakened after treated by electric field, which may be the reason that the bond strength of interface between the reinforcement and mortar decreased under the electric field.Corrosion state of steel plates in different environment were characterized by polarization curves, Electrochemical Impedance Spectroscopy(EIS) and XRD. The results show that the plates are easy to be rusted in neutral environment containing chloride, and large area of corrosion would take place in the early period. However, the corrosion process in high alkalinity environments were that the passive film were firstly spoilt by chloride, and some FeOCl generated in local area, then pitting corrosion beginned to occur and increasing progress.The corrosion state of steel plates with different initial corrosion during the current process were analyzed. The results show that the protection oxide layer would be regenerated on the surface of non-corrosion and pitting corrosion plates in electric field. The process were that the rust would firstly be dissolved, and the passive film could be regenerated on surface of steel plates. The protecttion layer would cover with the whole surface of steel plates to prevent corrosion with the passive film continually increasing..